"Zippy the Pinhead" wrote

Simple. If you want your kid to wear a helmet when s/he rides a bike,
you start getting the kid used to wearing a helmet when s/he rides the
trailer. And, of course, you model such behavior by wearing one
yourself while you're towing the trailer.

I never had the slightest problem with my kids. As a matter of fact, I have
a cherished snapshot of my (then) 2 year old (now 11) daughter sleeping in
her new helmet -- she wouldn't take it off.